Transparent optical communications systems use variable optical attenuators (VOA) for each wavelength channel to impose a relatively low frequency dither tone which is below the data channel in frequency and can be used to identify channels and their power levels in transmission. This VOA must be both sufficiently fast (e.g., 1 MHz), physically small and low cost, especially for applications with over 50 wavelengths in each system.
Some optical communications systems use bulk-silicon ridge waveguide VOAs, which are fast enough in attenuation response and relatively small. However, silicon waveguide VOAs use conventional fibre v-groove blocks to attach fibres on each side. These fibres are both large and costly relative to the size and cost of the silicon waveguide VOA chip. For reference, for optical splitters and arrayed-waveguide grating filters (both of which are high volume integrated waveguide devices), the integrated optical die tends to be less expensive than the fibre assembly used to attached the die to the fibres.
Another issue with silicon waveguide VOAs the use of a low-contrast waveguide design, which increases both the chip area (leading to lower wafer yield and higher cost) and requires fibre to be attached to both sides of the VOA (which increases the overall size of the component).